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great discrepancies of frequencies of the arbitrary waveform using niFGEN 5401

Hi all!

I'm using niFGEN PCI 5401 to generate an arbitrary waveform defined previously in the Soft Front Panel.
The reference guide says, that I can produce a frequency up to 1 MHz. Does this value refer to the whole waveform or to each sample of the 16384 samples? And how can you explain, that at last the arbitrary waveform takes about 600 ms for one cycle even though the frequency was set to 1 MHz?!

Thanks in advance
Steffi
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Hi Steffi

The PCI-5401 is a DDS based function generator instrument. DDS based waveform generation will not play every sample of an arbitrary waveform, and in fact, the samples will vary with the frequency selected.

The frequency will be in terms of the whole waveform. If you load one cycle of a sine wave in the 16314 samples, and select 1 MHz, you will see a one MHz sine wave on a scope.

To get an idea on how DDS works, there are two examples you can look at on NIDZ. They are found in the Fundamentals section under Signal Sources. Sorry, links may be to long, but you can browse NIDZ to them as well.

NI Developer Zone > Development Library > Measurement Hardware > Signal Sources > Fundamentals

LabVIEW Simulation of Direct Digital Synthesis ( DDS ) M
ode:
http://sine.ni.com/apps/we/niepd_web_display.display_epd4?p_guid=B45EACE3EEE956A4E034080020E74861&p_node=DZ52285&p_source=external

Direct Digital Synthesis Concept Example:
http://sine.ni.com/apps/we/niepd_web_display.display_epd4?p_guid=B45EACE3F03556A4E034080020E74861&p_node=DZ52285&p_source=external

Jerry
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Hi Jerry!

I generally understood how DDS works, but I'm bemused why the output frequencies do not correspond to the input frequencies?
1.) Yesterday I made a test with the GIVEN Soft Front Panel and even this does not work as it should: If I want a standard square function of 1 MHz, I get a square function of 2,5 Hz! (The same with 100 kHz = 0,25 Hz and 10 kHz = 0,025 Hz.) Where does this factor come from?
2.) This factor also corresponds more or less to the generated arbitrary function in my program: with 1 MHz I get 1,6 Hz, with 100 kHz = 0,16 Hz ...
3.) I thought changing the ratio between pulse length and length of the whole signal period (it's first a square and then a long time zero) I could generate a short pulse anyway accepting the mentioned f
requency discrepancies. E.g.: One period takes 600 ms (although my input frequency is 1 MHz). If I have a ration of 1 (square) to 100 (whole signal) I should get at least a pulse of 6 ms. But the shortest pulse I ever got was about 20 ms.
The IMPORTANT QUESTION: is there a physical limit for generating pulses (e.g. due to the inertia of the DDS method)?

I hope I could explain my problem more precisely. Thank you anyway!
Steffi
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Sorry, Jerry! I found the explication. It was a problem of the scope.
Steffi
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